1. Field of the Invention
The invention relates to power supplies utilizing pulse width modulated switching mode voltage regulation, particularly with respect to the output filter thereof.
2. Description of the Prior Art
Power supplies are known in the art utilizing switching mode voltage regulation ("Buck" Regulator). In such a power supply, the input AC voltage is converted to an input DC voltage by a rectifier section and filtered by an input filter. The filtered input DC is applied to a switching circuit that converts the filtered input DC to a pulse width modulated signal. The pulse width modulated signal is applied through an output filter to provide the regulated output voltage. The output voltage is fed back to the pulse width modulator to vary the duty cycle of the pulse width modulation in accordance with a comparison with a voltage reference. Thus, the duty cycle is varied to regulate the output voltage in accordance with the reference. The output filter comprises a smoothing filter to remove the carrier frequency of the modulation switching from the output voltage. In present day designs, it is often desirable to provide a high power, high component density, low bulk, low loss supply.
The output smoothing filter traditionally utilized in such voltage regulators is usually comprised of one or more inductor-capacitor (L-C) lowpass filter sections to remove the AC component introduced by the pulse width modulation and to provide a low ripple DC output. Each section of such a filter is comprised of a series inductor and a shunt capacitor. Generally, the output filter is designed with a cut-off frequency several decades below the fundamental switching frequency to provide adequate ripple rejection.
The design of the conventional output filter results in filter component values representing a compromise between the requirement to provide a sufficiently high value of filter inductance at a given direct current bias level, the physical bulk of the inductor and the resistive and frequency dependent losses of the inductor. Additionally, the filter capacitor should be selected from units that provide both a low series resistance at high frequency and a high capacitance value. The combined requirement for a relatively low cut-off frequency and low losses results in a substantially bulky unit. In high power, high density power supplies, the output filter generally occupies a significant percentage of the power supply bulk and contributes substantially to the power losses of the unit.
In addition, the low cut-off frequency required to provide adequate ripple rejection conflicts with the design requirements for the power supply error loop feedback control system. Additionally, the high phase shift introduced by multiple L-C filter sections introduces further difficulties into the feedback control system design. It is therefore appreciated that the design constraints of the conventional approach to switching power supply output filtering result in a single L-C section with a bulky filter inductor and capacitor or the use of multiple sections with an undesirably high phase shift.
The input inductor in such an L-C filter section has a high AC ripple current therethrough introducing significant losses. The large inductance required to obtain the low cut-off frequency requires numerous windings resulting in significant resistive loss. Additionally, the large inductance value requires the use of magnetic core materials to implement the inductor with a practical bulk. Conventional magnetic materials have a high self-heating loss when subjected to high frequency alternating ripple currents.
It is therefore appreciated from the foregoing that the smoothing output filter utilized in a conventional buck regulator is undesirably bulky and lossy introducing design difficulties in high density power supplies.
It is further appreciated that the conventional L-C output filter experiences undesirable variations in ripple at the switching frequency with variations in capacitance of the power supply load.